: And of course the wax seal broke and the subfloor is rotted. Is it possible that a leak occured, rotted the wood, then the toilet tilted which in turn tilted the flange?

: 1) The flange is very corroded on top. Do I need to replace it? How?

: 2)Can I modify the subfloor level in the bowl area to match the flange tilt?

: Thanks a lot for your help.

You just need to cut out the bad area of the floor and replace as needed. You don't say what material you have for the flange copper,cast iron, lead, PVC, or Abs. So i can't tell you how to change it. Call a licenced master Plumber, or you may be doing the job again in a year or two.Good luck on your project.